Insulin Dependent Diabetes Mellitus affects a substantial part of the human population. It is an autoimmune disease mediated by autoaggressive T cells. In our previous studies we have shown that TNFa expressed ectopically in the islets of Langerhans, accelerates and exacerbates the development of diabetes in NOD mice. The same transgene accelerates the development of diabetes in B6 mice with a CD80 transgene on their islets. In the previous funding cycle we established that the mechanism that underlies this exacerbation is the potentiation of presentation of islet antigen to autoreactive T cells. Regulatory mechanisms serve naturally to impede the progress of IDDM, and probably in many individuals prevent the development of disease. In the previous grant period we have found that regulatory CD4 T cells also develop in our models, and substantially slow the time course of diabetes. Prolonged expression of TNFa, however, overcomes the ability of these cells to prevent disease. In the present application we will elucidate which cells mediate the protective affects against TNFa exacerbated diabetes, the potency of these cells and the mechanism whereby they prevent CD8 T cells from promoting IDDM. We have also shown that these regulatory T cells depend on signals from CD40/CD154 and RANK/TRANCE. We will determine how these signals lead to the accumulation of Tregs in transgenic mice. It is likely that several overlapping regulatory mechanisms exist which together collaborate to inhibit autoimmunity. We have recently found that ICOS -/- mice are hyper-susceptible to the development of the autoimmune model EAE. We will determine whether this is also true for IDDM, and if so we will determine the mechanism whereby this occurs. The candidate mechanism immune deviation, for which we have evidence in the EAE system, will be explored in the current application. TGF beta has also been shown to inhibit IDDM. We will use two approaches to examine how TGF beta contributes to regulation of IDDM in our models. We will use regulated expression of TGF beta in the islets and a new T cell transgene which renders T cells resistant to inhibition to TGF beta to ask how TGF beta blocks IDDM and what happens in conditions when it cannot act.